As a technique of reproducing high density information recorded with a cycle beyond the resolution limit of the optical system in which recorded information is reproduced, super-resolution technology for optical recording media is being developed which involves a super-resolution layer on a medium in which microscopic marks or pits are reproduced. The super-resolution layer material is often a phase-change material.
For example, Patent Literature 1 discloses a method of forming a phase-change material layer on phase pits and liquefying a part of the phase-change material layer within a reproducing beam spot, thereby reproducing the phase pits corresponding to the resolution limit.
Patent Literature 2 discloses a method of providing a mask layer of Ge—Te alloy and forming in the mask layer a reproduction window having an increased optical transmittance, thereby reproducing recoded marks.
Patent Literature 3 discloses a method of providing an optical shutter layer containing three elements, Ge, Sb, and Te as main components and melting a part of the optical shutter layer within a reproducing beam spot, thereby reproducing recorded marks.
Patent Literature 4 discloses a method of providing a mask layer of Sb and irradiating the mask layer with a reproducing beam to form an optical aperture therein, thereby reproducing recorded marks.
Patent Literature 5 discloses a super-resolution reproducing method in which a mask layer is made of a dye material capable of supersaturated absorption.
As is understood from the fact that the prior art literatures refer to terms such as a mask layer, a reproduction window, an optical shutter, and an optical aperture, the conventional super-resolution reproducing method changes the optical properties of the super-resolution material in a fraction of a reproducing beam spot to reduce the effective beam diameter, thereby reproducing the microscopic marks or pits. One method of changing the optical properties of the super-resolution material in a fraction of a reproducing beam spot uses a phase-change material as the super-resolution material, in which a part of the phase-change material within a reproducing beam spot is melted.
The conventional super-resolution reproducing method is outlined hereafter with reference to FIG. 1. FIG. 1 is a cross-sectional view of an optical recording medium which contains a super-resolution material layer 101, a recording layer 102, a substrate 103, a recording mark or recording pit 104, an optical aperture 105 formed in the super-resolution material layer and a laser beam 106. Arrow 107 indicates the diameter of a beam spot and arrow 108 indicates the direction in which the laser beam is moved.
As shown in FIG. 1, in the conventional super-resolution reproducing method, an optical aperture is formed through a part of the super-resolution material within the beam spot diameter to reproduce a microscopic mark. The region within the beam spot diameter, except for the optical aperture, is shielded by the super-resolution material layer. The recording layer 102 is situated behind the super-resolution material layer 101 as seen from the light source 106. With the beam spot being blocked by the super-resolution material layer, the amount of light reaching the recording layer 102 is reduced, attenuating the intensity of the signals. When the recording mark or recording pit 104 is made smaller for higher recording densities, the optical aperture 105 accordingly must be smaller. Consequently, a fairly large part of the beam spot diameter is blocked by the super-resolution material, further attenuating the intensity of signals to the extent that the signals may not be detected.    [Patent Literature 1] Japanese Patent (JP-B) No. 3361079    [Patent Literature 2] Japanese Patent (JP-B) No. 3602589    [Patent Literature 3] Japanese Patent Application Laid-Open (JP-A) No. 08-306073    [Patent Literature 4] Japanese Patent Application Laid-Open (JP-A) No. 2000-229479    [Patent Literature 5] Japanese Patent Application Laid-Open (JP-A) No. 2003-157584.